NATURAL AND EXPERIMENTALLY ALTERED HYDRAULIC ARCHITECTURE OF BRANCH JUNCTIONS IN ACER-SACCHARUM MARSH AND QUERCUS-VELUTINA LAM TREES

The functional xylem anatomy and the hydraulic conductivity of intact and treated branch junctions of the diffuse-porous sugar maple (Acer s accharum Marsh.) were compared to those of the ring-porous black oak ( Quercus velutina Lam.). Maple shoots possessed greater growth intensit y than those of oak. The extensive growth of the maple trees resulted in about a two-fold increase in xylem production in the maple branches . Branches were altered by removing a patch of bark from the base of a branch (near a junction) leaving a bridge of bark on the upper or low er side of the branch. The experimentally treated branch junctions rev ealed that, in oak, most (up to 92%) of the water flows in the lower s ide of a branch, where most of the large vessels occurred. In maple, m ost of the conductive tissue was observed to form in the upper side of the branches, which was equally or more conductive than the lower sid e. A treatment of longitudinal, parallel scratches in the bark-bridge, which reduced earlywood vessel width, substantially decreased conduct ivity (to only 15%) in oak, but had no effect on conductivity in maple . In maple, such wounding stimulated more wood formation and increased conductivity. In both trees, a narrow bridge at the junction induced more wood formation and higher conductivity in the branch. The mechani sms controlling wood formation and water flow in branch junctions of r ing- and diffuse-porous trees are discussed.